Trying to match a colour exactly, I am left wondering what exactly is the difference between Opacity and Alpha.

On ordinary paper, my colour is colour a5bbff, alpha 255, opacity 70%. For the finished article, I am printing on thickish, heavy white paper, I told the printer it is glossy photo paper, which it prints much more slowly and carefully, but then my blue came out too dark. Using the colour wheel dialogue, I set colour a5bbff, alpha 140, opacity 60%. Then I get exactly what I want on that paper and paper setting.

Since Alpha and Opacity both seem to relate to the opacity, what is the difference? Why have both?

Opacity controls the entire object, including pattern fill or a bitmap image, alpha controls the color (fill and/or stroke).

Flashback...crowd...little lady...celebrating...


Because that way you can lower the opacity of an object, group of objects, and therefore layers as well.

Because that way you can draw markers on a path not showing it's stroke.


Because oh come on, why don't we have both?



On the colour matching, printed colours usually appear darker than on screen -note that you didn't printed on a backlit paper, the only lightness coming from the paper is reflected light. Could it be matched exactly with emitting pixels on screen?
If you calibrate your screen from time to time with professional tool, you might get closer result. Probably it would turn your screen darker?


Regarding opacity and alpha values, pdf-s are rendered on a white background by default. Maybe it would be a good idea not to use any kind of transparency in your printing material.

Thanks! So since my object is just a block of solid colour, no stroke, I could get the same effect by leaving the opacity at 100% and playing with the Alpha? Just curious.

Alpha values go from 0 to 255, opacity from 0 to 100%. You cannot match all the values of the two scales precisely, for example a 70% opacity would need a 178,2 alpha value.